Divine command theory

Divine command theory suggests that any statement about ethics is actually a statement about the attitudes and desires of God. That is, it claims that God's commands and morality are identical. To suggest that morality can exist without God is therefore a contradiction. Divine command theory is a form of theological voluntarism.

Criticism

Begging the question

Divine command theory cannot prove that God is the source of morality because that is precisely what it assumes. That is, divine command theory assumes that whatever God commands must be moral (in fact, in most cases it defines morality that way). However, it's not clear that I am morally required to do something just because God commands it. I might want to obey God in order to escape punishment, but this is a matter of my own selfish interest and not an objective moral obligation. Similarly, it's not clear why I should assume that there's no other possible source of morality.

Unless divine command theory can first demonstrate that it is the most appropriate view of ethics, one cannot assume that it is correct to prove anything else.

Non-standard usage of the words "good" or "moral"

Most people have an intuitive sense of what it means for an action to be good or to have a moral obligation, and this set of moral attitudes typically pre-dates or is independent of any religious beliefs. To define a new meaning for "morality" as meaning what God wants, then to act as if this is the same as the everyday conception of morality, is to commit an equivocation fallacy. Morality is either a system for determining which actions are right or wrong, or a desire to obey the will of God. It can't mean both things at the same time, unless one first demonstrates that both meanings are equivalent.

Divine command theory is not an objective system of morals

Divine command theory implies that whatever God commands must be the morally correct course of action. Therefore, if/when God endorses genocide, infanticide, animal sacrifice, slavery, or rape, those things are good, whereas if/when he forbids eating certain foods or working on certain days or having certain kinds of kinky sex, those things immediately become bad. This makes divine command theory a subjective theory of morals, one which is arbitrary and can change at God's whim.

One way of countering this argument is to say "God wouldn't do that", but this doesn't help at all. For one, in many religious traditions he does do such things. For another, if God is the source of morality, he can do whatever he wants and it would still be just as "good" as anything else.
Another thing to note is that many apologists claim that the god they speak of is unpredictable and works in mysterious ways; By their own claim, they cannot warp their "flawed human reasoning" around what is beyond it to anticipate what said go would or would not carry on: an example of the biblical scriptures is Abraham being unflinchingly convinced that God's commandments were inherently good even when said deity commanded him to sacrifice his son Issac. The lamb appearing before the sacrifice, which Abraham didn't expect at all, doesn't change the apparent fact that the later saw that his god's orders as undeniably good, even if it would have ended with his son's slaughter. It would seem that their idea of something that is "bad, and my god wouldn't do it" is derived from a more personal moral code, unrelated to said religion, made all the more apparent since people interpreted passages differently throughout history, like when some justified the enslavement of Afro-Americans an era ago using a few biblical passages.

Thomas Aquinas believed that God's commands come from his own (unchanging?) essence and thus were not arbitrary pronouncements. This is irrelevant to the problem. Either there is a single objective, necessary code of morals that governs everything, in which case God's commands merely reflect (or fail to reflect) this standard, or else there is no such code, and so the commandments of God cannot reflect an objective morality. Either way, it gets you nowhere to say that actions are good for no other reason than because God approves of them.

"God is good" becomes meaningless tautology

Theists describe God as good and loving, which is problematic primarily due to the problem of evil. But setting that aside, if goodness is defined as Godly, then "God is good" is an empty statement, reducing to "God only acts in accordance with the ways God acts." Yet theists almost never treat "God is good" as a tautology. For example, Christians say that God-as-Jesus was being good and loving by sacrificing himself to save humankind from the wages of sin. Yet under divine command theory, God would have been exactly as good if he never sacrificed himself, or if he decided to send everyone (Christian or otherwise) to suffer eternally in Hell, or if he put everyone in Heaven, or if he turned everyone's legs into tree trunks. One cannot point to anything God does as an "example of" or "evidence for" God being good, because there is no hypothetical action God could take that, if he did it, would not be an action God takes, and therefore not be "good" – indeed, maximally good – by divine-command standards.

People are told to have faith in God and to demonstrate this by, for example, praying. Yet if God were to personally appear to a loyal petitioner (someone asking for God to help a sick child, say) and tell him/her that he is now sick of prayer and that he's going to punish the child out of spite, then that would be the maximally good acton for God to have taken. Thus, extrapolating back to the present, there is no reason to trust God to do that which we humans might consider good, since his actions cannot be bound by a moral system outside himself. Even if he promises to act a certain way, his breaking the promise later would (since he did so) be the right thing for him to do.

Divine command theory cancels out most theodicy. For example, a common theodicy is the free-will defense – evil must be permitted because otherwise humans would be robots, which would be bad. Apart from the other issues with this argument, it doesn't fit in the same box as divine-command theory. After all, if God did make us all robots, he would remain 100% good.

Divine command theory is impractical

Whether divine command theory is true or not (and there seems to be no reason to think that it is), it is often not an effective method of settling moral dilemmas. For one, it's not clear which religious tradition is correct. For another, religious texts tend to contain many conflicting, arbitrary, or excessively specific rules. These rules rarely allow a clear method of generalizing these ideas to every possible situation, so a believer is forced to do much the same thing that an atheist does, which is to work out moral principles and ideas for herself. Often, the fact that the believer is bound to respect certain statements as absolute truth makes this process even harder, because those statements may not make good sense, or may make sense in most situations but be absurd in others. Divine command theory thus fails to provide moral guidance for much the same reason that religions often fail to provide moral guidance.

Morality follows from the nature of God

Catholic theology considers morality to follow from the nature of God, not the will of God. This concept is known is natural law.